DE3520699A1 - METHOD FOR SELECTIVE DIFFUSING ALUMINUM INTO A SILICON SUBSTRATE - Google Patents

Description

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7.6.85 -X- Rz / dh

Method for the selective diffusion of aluminum into a silicon substrate

The invention is based on a method for the selective diffusion of aluminum into a silicon substrate according to the preamble of the patent claim

Aluminum, which has a large diffusion constant in silicon is one of the most important dopants for the production of power semiconductor components like diodes and thyristors. Various methods are used for aluminum diffusion:

a) Diffusion in a vacuum, in which the diffusion tube itself serves as the aluminum source. The diffusion tube consists of silicon, is doped with aluminum and housed in a conventionally evacuated quartz glass tube .

b) A diffusion in argon as a protective gas, in which a second silicon wafer serves as the aluminum source, those with aluminum through migration of the Al-Si melt is doped, and in which the source disc is in direct contact with the target disc.

c) A diffusion in which the aluminum is applied to the target _{in the form of an Al (N0 3} ) _{5 layer.}

All the methods mentioned have one thing in common, aside from others Disadvantage: Selective diffusion is not possible here. In the method according to b), one-sided Al diffusion is possible, experiments with a diffused structure however, have failed on a disk surface.

For edge contouring, i.e. the shaping of the edge of a semiconductor component, as is required for high-voltage components, it is advantageous if the Blocking PN junction on which the entire voltage is in the blocking direction, is pulled lower at the edge or if a so-called guard ring can be formed. A deep PN transition is required for field-controlled thyristors to diffuse, for which a deep selective P diffusion is necessary. In terms of the required Penetration depth of this diffusion is practically only aluminum possible as a dopant, since boron or gallium have too low a diffusion constant. The diffusion constant of aluminum is about 10 times as large as that of boron. Gallium diffuses a little faster compared to boron, but it cannot be masked. The problem of selective Al diffusion has not yet occurred has been satisfactorily resolved. In practice, this task is carried out in such a way that after the occupancy (predeposition) the silicon is etched in the undesired areas down to the depth of penetration of this diffusion. Then find the second diffusion stage (drive-in) takes place in such a gas mixture that the out-diffusing aluminum binds.

With the preamble, the invention takes on a state of the art of methods for selective diffusion

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of aluminum in a silicon substrate, as described in DE-AS 2,812,658. There, aluminum is deposited from an aluminum wire with a purity of 99.9995 % as an aluminum source with a thickness of less than 2 μm in a lattice-like pattern on a main surface of a single crystal silicon semiconductor substrate. The aluminum wire is heated by an electron beam and the aluminum at a pressure in the range of 270 to 400 uPa uPa on to 230 ⁰ C - 250 ⁰ C deposited heated silicon substrate and photolithographically patterned in a conventional manner. The Si substrate is heated in an atmosphere of a non-oxidizing gas such as nitrogen or argon and 0.05-10% by volume of oxygen until a coherent oxide layer is formed on the aluminum and silicon surface. The disadvantage here is that the area around the irradiated aluminum wire also rises to a high temperature due to the electron beam, which can lead to evaporation, for example, of crucible material and thus to undesirable contamination of the aluminum vapor deposition layer.

Difficulties arise when the aluminum-coated silicon wafer is heated because the Al-Si melt is not evenly distributed because of an intermediate layer on the Si surface, which turns out to be Shows formation of small droplets. As a result, the homogeneity of the diffusion is not guaranteed.

The invention as defined in claim 1 solves the problem with a selective aluminum diffusion to achieve a higher purity of the aluminum on the Si substrate or the proportion of impurities

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Reduce the effects of foreign atoms, especially heavy metal atoms.

One advantage of the invention is that contamination-free aluminum layers are applied to the surface of the Silicon wafer can be applied.

According to an advantageous embodiment of the invention it is achieved that these aluminum layers adhere well and evenly melt together with the silicon when heated, so that no droplet formation occurs.

For the relevant prior art, reference is also made to US Pat. No. 4,199,386, from which it is known to apply a complex structure made up of several layers to the silicon wafer, which is then structured and serves as a diffusion source.

The invention is explained below using an exemplary embodiment. Show it:

1 shows an arrangement for aluminum coating a silicon wafer,

Fig. 2 shows a silicon wafer according to FIG. 1, which is made with aluminum and photoresist is coated,

3 shows a silicon wafer according to FIG. 2 after the aluminum has been etched away,

4 shows a silicon wafer according to FIG. 3 after the photoresist has been removed,

5 shows a silicon wafer according to FIG. 4 after diffusion in an oxygen-containing atmosphere and

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6 shows a diffusion profile of the silicon wafer according to Fig. 5 below the aluminum layer.

In Fig. 1, 1 is a silicon wafer or a silicon substrate denotes whose upper major surface 2 is polished and is to be coated with aluminum.

The silicon wafer 1 of the η-conductive type has a diameter of 50.8 mm and a specific resistance in the range of 40 Λ cm - 60 / lern. She lies on one Washer 3 made of silicon, so that it is in a commercially available during the aluminum coating High vacuum cathode sputtering system with the turntable 4 does not come into contact. 5 denotes an aluminum baffle plate or an Al target that is connected as a cathode and is sputtered. This Al target is shielded by a shielding electrode 6.

The Al target 5 is atomized by ion bombardment in the plasma of a noble gas, preferably in an argon plasma and an aluminum covering or an aluminum layer 7 of predeterminable thickness of, for example, 1 µm on the silicon wafer deposited, see Fig. 2. It is of great importance that the Si surface is prior to sputtering Ar ions are bombarded for about 10 minutes at 20 pbar argon pressure, which removes the silicon layer and the SiO "skin (native oxide) located on it. This creates direct contact between Al and Si is guaranteed without the interfering separating layer, and this applied Al layer melts with the silicon evenly together.

If the aluminum layer 7 would only be dusted on without prior removal by bombing the main surface 2 with Ar ions, it could be in a subsequent

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the diffusion lead to an undesired formation of droplets of the aluminum on the main surface 2, small Areas of the main surface 2 remain without an aluminum layer.

The atomization takes place at an argon pressure of 8 ubar with a deposition rate of 340 pm / s up to a layer thickness of 1 μm. It is in comparison to the Vapor deposition is a relatively cold process, which contributes to the cleanliness of the applied substance, since there is no boat or crucibles, which are a source of contamination, are heated. Soiling from the turntable 4, on which the silicon wafer 1 rests in the sputtering system, can by placing the a little larger silicon washer 3 can be prevented. This washer 3 has a in the middle Recess in which the silicon wafer 1 to be treated lies and cannot slip away.

After the sputtering, the aluminum layer 7 is photolithographically masked in the usual way and a strip-shaped one Photoresist coating 8 applied. Photoresist is e.g. under the designation AZ-1350 J-SF from the German Company Höchst available. It is useful to have a photoresist coating on the back of the silicon wafer 1 as well 9 to apply in order to prevent the silicon wafer from contaminating during subsequent manipulation will.

After the application of photoresist coatings 8 and 9, the aluminum is in a "ISOFORM" aluminum etchant MIT semiconductor Chemie GmbH, Solingen, Germany, 30 s etched at 55 ⁰ C long, so that the position shown in Fig. 3 Structure emerges.

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The photoresist that is no longer required is then removed removed with acetone and isopropanol for 15 s each in an ultrasonic bath and the silicon wafer 1 rinsed with water and dried, so that the structural door shown in FIG arises.

The adhesion of the aluminum can now be ^{further improved by tempering at 480 °} C.-500 ^° C. for 30 minutes in a vacuum; However, it is of minor importance compared to an Ar bombardment, which experiments have confirmed.

The subsequent diffusing of the aluminum covering 7 in the silicon wafer 1 is performed 300 min at a temperature of 1250 ⁰ C in a gas mixture of _N2 (1 L / min) and 0_ (20 ml / min), wherein the heating rate is 3 K / min . The cooling takes place at a cooling rate of 1 K / min.

The oxygen has the task of converting the aluminum vapors from the aluminum layer into Al _{2 O.} Al ₂ O ^is ineffective as a diffusion source and cannot produce a P-layer on undesired areas. Argon can also be used instead of nitrogen. The areas in which no diffusion should take place can additionally be provided with an SiO ₂ layer as a diffusion barrier against the other dopants that are present in the tube or already in the silicon wafer.

After the diffusion, the remnants of Al, Al ₂ O ,, melt made of Al-Si and possibly SiO _{2 can be} etched off and lapped off. The abrasion during polishing is small compared to the abrasion caused by the Al pre-allocation.

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If a lower edge concentration is desired, can now use a so-called drive-in diffusion be connected.

During diffusion, aluminum atoms diffuse up to a diffusion depth d into the interior of the silicon wafer 1 and there form a p ⁺ - doped zone 10 in the η-conductive silicon, see FIG. 5.

The resulting diffusion profile under an aluminum layer 7 is shown in FIG. 6, in which the penetration depth a of the aluminum atoms in μm is plotted on the abscissa and log η, η = Al concentration = number of Al atoms / cm ³ , on the ordinate. With increasing penetration depth a from the main surface 2, the concentration η decreases. No P-doping was found in the areas without aluminum layer 7.

Further possible applications can be all processes in which the silicon is melted evenly with the aluminum is important, which is the case, for example, with migration.

Claims (6)

58/85 claims 1. Method of selective diffusion of aluminum into a silicon substrate, a) wherein on at least one main surface (2) of the silicon substrate (1) deposited aluminum, b) this aluminum is structured according to a predeterminable pattern and c) the silicon substrate coated with an aluminum pattern in one of a non-oxidizing one Gas and oxygen existing atmosphere is heated to a diffusion temperature, characterized, d) that the aluminum is deposited on the silicon substrate by cathode sputtering in a non-corrosive gas plasma is deposited. 2. The method according to claim 1, characterized in that a) that the atomization in a noble gas plasma, b) is carried out in particular in an argon plasma. 3. The method according to claim 1 or 2, characterized in that a) that the silicon substrate (1) before the aluminum deposition with noble gas ions, b) is bombarded with argon ions in particular. 4. The method according to any one of claims 1 to 3, characterized marked, a) that the non-oxidizing gas when heated is nitrogen and / or b) is argon. I 58/85 5. The method according to any one of claims 1 to 4, characterized marked, a) that on at least one main surface (2) of the silicon substrate coated with an aluminum pattern (1) a diffusion protective layer for others before heating to the diffusion temperature Dopants are applied as aluminum, b) in particular that the diffusion protective layer is an SiO "layer. C. 6. The method according to any one of claims 1 to 5, characterized in, a) that the silicon substrate (1) is heated from approx. 500 ^° C. to a diffusion temperature ^{of approx. 1250 °} C. with a heating rate of approx. 3 K / min, b) that the silicon substrate is kept at this diffusion temperature for about 300 minutes and c) is then cooled ^{to about 500 °} C. at a cooling rate of about 1 K / min.